Investigating the cause of late deformity following fronto-orbital remodelling for metopic synostosis using 3D CT imaging

Abstract

Late deformity/indentation is well-recognised following fronto-orbital remodelling (FOR) for metopic synostosis. We hypothesise that if damage to temporalis muscle was a contributor, thickness of soft tissue and bone in the affected area would be reduced.Soft tissues and bone were separately segmented and 3D reconstructed from CTs of 8 patients 1.5-18 years post-FOR performed at 16±2 months for metopic synostosis and from 8 agematched controls. Soft tissue (taken as proxy for temporalis muscle) and bone thickness overall and in the indented areas were computed.Post-FOR, 3D soft tissue thickness maps demonstrated temporalis extending upwards but falling short of the indented area. Overall skull thickness increased with age post-FOR (logarithmic fit R 2 = 0.71) and for controls (R 2 =0.90). Although immediately post-FOR the future indented area had a thickness of 98% of control, it decreased linearly to 64% 16 years later (Pearson's r=0.84).These findings suggest that late post-FOR deformity/indentation is enhanced by limited upward extension (or retraction downwards) of temporalis muscle while bone thickness in the affected area gradually decreases. This supports the hypothesis that aberrant re-attachment of the temporalis muscle makes a material contribution to late deformity following FOR for metopic synostosis.

Late Deformity Following Fronto-Orbital Reconstructive Surgery for Metopic Synostosis: The Role of Temporalis Muscle

“…19 It is this component that has been shown to have soft tissue and bony elements with thinning of both. 3 The present study strongly supports the theory that the soft tissue contribution to this component of LDD is the absence of the superior elements of temporalis from their normal position. This possibility has been raised before but discounted on the evidence of parental assessments of muscle location by palpation during their children's jaw movements.…”

“…A previous study 3 demonstrated thinning of bone and soft tissue in the most affected area, the latter being attributed to absence of the upper elements of temporalis from their usual location. Possible explanations included per-operative damage to the muscle’s blood supply, 4,5 its innervation 6 and/or its fatpads, 7,8 inadequate muscle resuspension or its subsequent retraction 9,7 and, finally, interference with a previously undescribed childhood expansion of the muscle’s origin.…”

Observations on the growth of temporalis muscle: A 3D CT imaging study

“…The absence of these superior elements may also contribute to its bony component by the removal of the muscle's normal osteogenic stimulus (Rodriguez-Florez et al, 2019).…”

“…The investigation was prompted by a previous study (Rodriguez‐Florez et al, 2019) into the late‐developing deformity that can complicate reconstructive operations carried out between 6 and 18 months of age for metopic synostosis (van der Meulen et al, 2008; Wes et al, 2014) that identified the absence of temporalis in the area normally occupied by its superior elements as the prime contributor to its soft tissue component. We hypothesized that this deformity (which typically becomes apparent some 3–4 years post‐surgery) was the consequence of surgical interference (the operations involve detachment and later re‐attachment of temporalis) with a previously undescribed childhood upward extension of the muscle by operations performed between 6 and 18 months of age.…”

“…Agreement between two observers acting independently was higher for the measurements of the muscle's vertical reach as both found the anteroposterior (horizontal) boundary of the muscle harder to define on the 3D CT scan lateral projections used for this investigation. No significant differences were found between right and left sides or between male and female in the 50 children studied.The investigation was prompted by a previous study(Rodriguez- Florez et al, 2019) into the late-developing deformity that can complicate reconstructive operations carried out between 6 and 18 months of age for metopic synostosis(van der Meulen et al, 2008;Wes et al, 2014) that identified the absence of temporalis in the area normally occupied by its superior elements as the prime contributor to its soft tissue component. We hypothesized that this deformity (which typically becomes apparent some 3-4 years post-surgery) was the consequence of surgical interference (the operations involve detachment and later re-attachment of temporalis) with a previously undescribed childhood upward extension of the muscle by operations performed between 6 and 18 months of age.…”